Automatic gain control



4, 1938. A. J. H. VAN DERVEN 2,104,107

AUTOMATIC GAIN CONTROL I Filed Feb. 18, 1931 INVENTOR ADRIANUS 1. H. VandenVEN ATTORNEY Patented Jan. 4, 1938 UNITED STATES PATENT OFFECE2,104,107 A 'roMATIo GAIN CONTROL Application February 18, 1931, SerialNo. 516,539 In the Netherlands April 15, 1930 7 Claims.

This invention relates to wireless receiving cir' cuit arrangement withautomatic volume con trol. Automatic volume controls are known, and arefrequently used in present day radio receiving sets. In most cases aseparate rectifier is used in the output circuit of which a resistanceis connected, and Whose input circuit receives the high frequencyoscillation which may be amplified, or not. According to the strength'ofthe signals being greater or smaller, a stronger or weaker'directcurrent is set up in the resistance connected intothe output circuit ofthe rectifier. This direct current causes a drop of potential in theresistance, and this drop is used for regulating the negative grid biasof one of the high frequency amplifying tubes. Furthermore, it hasalready been proposed to control the mega tive grid bias of one of thehigh frequency amplifying tubes by means of resistance connected in theplate circuit of the detector tube.

According to mypresent invention the negative bias in one of the highfrequency amplifying tubes is obtained by connecting the'grid of thedetector tube, preferably across a high resistance, to a point situatedat the filament side of the grid circuit of one of the high frequencyamplifying tubes. Between this point and the cathode of the highfrequency amplifying tube in question a condenser is inserted, by whichthe production of undesired coupling effects is prevented, and,

in addition, the connection between the grid of the detector tube andthe cathode of the high frequency amplifying tube is blocked for directcurrent.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically one circuit organization whereby my invention may becarried into effect.

The figure shows a receiving antenna I which, by means of a smallcondenser 3, is coupled with the tuned circuit 2, the latter beingconnected to The source of heating current is denoted by Instead of thebattery shown on the diagram, another source of current, for example atransformer connected to a commercial power line, may naturally be used.Into the plate circuit of the detector 9 a telephone IE, or other loadresistance is connected. The resistance !2, which may also be replacedby an impedance of a more general nature, is so chosen that the grid ofthe detector tube 9 is not unfavorably affected by the presence of thisresistance, or impedance. The condenser I3 serves at the same time toprevent any undesired coupling between the grid circuit of the detectortube and that of the amplifying tube 4.

Furthermore, care should be taken, of course, that the grid bias of thetube 4 Which is regulated by the connection provided according to theinvention, is independent of the low frequency voltage variations of thegrid of the detector tube. This may be easily obtained by a properchoice of the magnitude 'of the resistance l2. The circuit arrangement,according to the invention, may be easily constructed, and requires onlya small capacity of the condenser is, since there is no objection ingiving the resistance I 2 a very high value (for example 1 to 2 bcgohms,or still more).

When applying the known device above referred 'to, in which the gridbias is obtained from a resistance placed in the plate circuit of thedetector tube, then in order to prevent undesired coupling with the gridcircuit of the amplifying tub-e 4, a much larger capacity'must becoupled in parallel to the resistance from which the grid bias isobtained.

Of course,'the invention may be also applied -to circuit arrangementscomprising tubes having indirectly heated cathodes. The modificationsrequired in this case in the diagram of connections may be easilyprovided by those skilled in the art. Again, while I have indicated anddescribed one arrangement for carrying my invention into effect, it willbe apparent to one skilled in the art that my invention is by no meanslimited to the particular organization shown and described, but thatmany modifications may be amplifier tube to impress upon the amplifiergrid a potential developed by grid current flowing through the detectorgridleak.

2. In combination, in a radio receiver including a radio frequencyamplifier stage provided with a tube having a tunable input circuit anda detector stage provided'with a tube having a tunable input circuitcoupled to the output circuit of the amplifier tube, an electricallyconductive path, including an impedance of a predetermined magnitude,connected between the grid of said detector tube and the grid of saidamplifier tube, a condenser connected between the detector grid and thehigh potential side of the detector input 3 circuit, a grid leakresistor connected between the grid and cathode of the detector tube,said path being connected to the detector grid at a point intermediatethe condenser and the grid terminal of said resistor.

3. In combination, in a radio receiver including a radio frequencyamplifier stage provided with a tube having a tunable input circuit andadetector stage of the grid leak type provided with a tube having atunable input circuit coupled to the output circuit of the amplifiertube, an electrically conductive path, including an impedance of apredetermined magnitude, connected between the grid of said detectortube and the grid of said amplifier tube to impress upon. the amplifiergrid a potential developed 'by grid current flowing through the detectorgrid leak, saidimpedance consisting of a resistor having a magnitudesuch that the grid bias of the amplifier grid is independent of the lowfrequency voltage variations of the detector tube grid. 7 a

4. In a signalling system including an amplifier having a vacuum tubeprovided with a control grid, and operating to amplify signals atfrequencies above audibility, and a grid-leak detector, the method ofcontrolling the degree of amplification in said amplifier whichcomprises utilizing current flowing through said grid-leak to vary thebias on the control grid of said amplifier.

5. In a signalling system including an amp1ifier having a vacuum tubeprovided with a control grid and operating to amplify signals aboveaudibility, and a grid-leak detector, the method of controlling thedegree of amplification in said amplifier which comprises filteringcurrent flowing through said grid-leak and utilizing the filteredcurrent to control the bias on the control grid of said amplifier.

6. In a carrier wave receiving system, one or more carrier frequencyamplifying tubes, a detector tube having a grid-cathode input circuitand a cathode-anode output circuit, means for deriving a uni-directionalvoltage from the grid rectification action of said detector tube, thevalue of said voltage depending on the carrier first voltage, and meansfor applying said first voltage to at least one of said carrierfrequency amplifying tubes, whereby the efiiciency of said tubes ismodified in accordance with the carrier 'wave amplitude. V

7. In a modulated carrier Wave receiving system, at least one modulatedcarrier wave transmission tube, a detector tube having at least acarrier wave input electrode, a cathode and an anode, a modulatedcarrier wave input circuit connected between the input electrode andcathode, and an output circuit connected between the anode and cathode,means for impressing carrier wave energy transmitted by saidtransmission tube upon said detector input circuit, means for deriving auni-directional voltage from the grid rectification action of saiddetector tube, the

value of ,said voltage. depending on the carriervoltage, and means forapplying said uni-directional voltage to said transmission tube wherebythe efficiency of the tube is modified in accordance with the carrierwave amplitude.

ADRIANUS J OHANNES HEINS VAN DER VEN.

